What is a deadlock?

Question

When writing multi-threaded applications, one of the most common problems experienced are deadlocks.

My question to the community, is:

What is a deadlock? How do you detect them? Do you handle them? And finally, how do you prevent them from occurring?

Answer 1

Once again, Jeff has the answer. ;-)

Answer 2

A lock occurs when multiple processes try to access the same resource at the same time.

One process loses out and must wait for the other to finish.

A deadlock occurs when the waiting process is still holding on to another resource that the first needs before it can finish.

So, an example:

Resource A and resource B are used by process X and process Y

• X starts to use A.
• X and Y try to start using B
• Y 'wins' and gets B first
• now Y needs to use A
• A is locked by X, which is waiting for Y

The best way to avoid deadlocks is to avoid having processes cross over in this way. Reduce the need to lock anything as much as you can.

In databases avoid making lots of changes to different tables in a single transaction, avoid triggers and switch to optimistic/dirty/nolock reads as much as possible.

Answer 3

Deadlock occurs when two threads aquire locks which prevent either of them from progressing. The best way to avoid them is with careful development. Many embedded systems protect against them by using a watchdog timer (a timer which resets the system whenever if it hangs for a certain period of time).

Answer 4

A deadlock happens when a thread is waiting for something that never occurs.

Typically, it happens when a thread is waiting on a mutex or semaphore that was never released by the previous owner.

It also frequently happens when you have a situation involving two threads and two locks like this:

Thread 1               Thread 2

Lock1->Lock();         Lock2->Lock();
WaitForLock2();        WaitForLock1();   <-- Oops!

You generally detect them because things that you expect to happen never do, or the application hangs entirely.

Answer 5

Deadlocks will only occur when you have two or more locks that can be aquired at the same time and they are grabbed in different order.

Ways to avoid having deadlocks are:

• avoid having locks (if possible),
• avoid having more than one lock
• always take the locks in the same order.

Answer 6

A deadlock is a state of a system in which no single process/thread is capable of executing an action. As mentioned by others, a deadlock is typically the result of a situation where each process/thread wishes to acquire a lock to a resource that is already locked by another (or even the same) process/thread.

There are various methods to find them and avoid them. One is thinking very hard and/or trying lots of things. However, dealing with parallelism is notoriously difficult and most (if not all) people will not be able to completely avoid problems.

Some more formal methods can be useful if you are serious about dealing with these kinds of issues. The most practical method that I'm aware of is to use the process theoretic approach. Here you model your system in some process language (e.g. CCS, CSP, ACP, mCRL2, LOTOS) and use the available tools to (model-)check for deadlocks (and perhaps some other properties as well). Examples of toolset to use are FDR, mCRL2, CADP and Uppaal. Some brave souls might even prove their systems deadlock free by using purely symbolic methods (theorem proving; look for Owicki-Gries).

However, these formal methods typically do require some effort (e.g. learning the basics of process theory). But I guess that's simply a consequence of the fact that these problems are hard.